(this is all assuming that you live in the states)

Do either of your parents have health insurance that you are covered under (or your own)? If so, good. If not, I don't know where you live, but you can use the google machine and look up mental health facilities in your area. Find the publicly funded locations. These places are generally staffed with advocates who will help you obtain healthcare and get you set up to see a doctor.

Would they support you, or allow you to see a professional? If so, good. If not, but you do have healthcare coverage, use the google machine to search for facilities and offices and call around, check if they accept your insurance and set up an appointment on your own. You're 18, you don't need permission from your parents, nor do you have to disclose to them, all you would need is your insurance information.

See a professional. Being 18 can mean a lot of different things in regards to what may or may not be going on in your head. With Bipolar, most diagnosis come in the mid 20's, some sooner, some later, and all for a myriad of reasons. Maybe it's this thing, maybe it's that, maybe you would benefit from medication, maybe you wouldn't. But the best thing that you could do in your situation is to see a professional. A first step could even be just going to see your PCP (Primary Care Physician, or the guy/gal you've been going to for shots and checkups through your life). If you're parents are resistant, even if they are the type to insist on taking you to see the doctor, you can schedule a physical and silently request that they not be present for the exam so you can actually talk 1 on 1. It is your legal right to keep matters of this nature between you and your doctor.

Be honest and forward. Tell your doctor about all the feelings that have been affecting you. Give them your assessment (avoid self prescribed diagnoses).

tl;dr: See a professional.